One reason some women may be hesitant about getting the Covid vaccines is because of concerns of it changing their menstrual cycle. While the CDC gathers information on Covid vaccine side effects, they do not actively collect data on changes to women’s menstrual cycle. Recent research suggests Covid vaccination may cause short-term changes to your menstrual cycle, which, if you experienced a late or heavy period after getting your shot, may make you feel validated. A new study published in the journal Obstetrics & Gynecology found Covid vaccines may cause minor delays in your period. The study also found that these changes are temporary and go away after one or two cycles.

While a late period is great news for women looking to get pregnant, it can be a cause of panic for others. But not all late or missed periods are a sign of pregnancy. About 14% to 25% of women experience irregular menstrual cycles from the time they get their first period to when they reach menopause.

There are several reasons why a period may be late. Travel, exercise, stress or other changes in routine can cause cycle fluctuations, as can taking emergency contraceptives, or starting perimenopause. 

But the pandemic can also be to blame. Even if you’ve never been affected by a Covid infection, menstrual cycles have been shown to be affected by the stress of the pandemic (we probably don’t need to tell you about that). The current study adds to the need for more research on the impact the Covid pandemic has had on women’s health.

Covid vaccines linked to a delay in menstrual cycles

Scientists have been closely following any mild or severe Covid vaccine side effects people have reported on the CDC’s Vaccine Adverse Event Reporting System (VAERS). Most people reported side effects such as headaches and muscle aches, but a growing number of women have drawn concern over changes to their menstrual cycle.

So a group of researchers led by Alison Edelman of Oregon Health and Science University collected data on the menstrual cycles of women vaccinated with Covid vaccines from October 2020 to September 2021.

Before vaccination, the women in the study had an average menstrual cycle of 24 to 38 days. More than half opted for the two-dose Pfizer-BioNTech Covid vaccine, while 35% received Moderna and 7% received the Johnson & Johnson one-dose shot.

The study examined the menstrual cycles of 2,403 vaccinated women and 1,556 unvaccinated women. There were some differences between the vaccinated and unvaccinated group. Compared to unvaccinated women, 34% of vaccinated women were 30 to 34 years old. Other differences between groups was that vaccinated women were more likely to be college-educated, identify as white and had not given birth to a child. 

Vaccinated women showed slightly longer period cycles compared to unvaccinated women. On average, women reported that their first menstrual cycle after being vaccinated was delayed by almost half a day. For vaccinated women who received a second dose, their periods came nearly one day later than scheduled. However, Covid vaccines did not affect how long women bleed during their periods. 

The effects of getting two doses in the same menstrual cycle

According to the researchers, the increased delay in periods may be because some women received both vaccine doses within the same menstrual cycle. Their results found a 2-day increase in their menstrual cycle after vaccination.

About 10.6% of women who received both doses in the same menstrual cycle experienced a delay of 8 or more days compared to 4.3% of unvaccinated women. However, the delayed periods after vaccination seem temporary. Researchers found changes to the menstrual cycle went away after two cycles.

Why COVID-19 vaccines may affect menstrual cycles

While not directly explored in the current study, the research team hypothesizes that Covid vaccines create an immune stressor that potentially alters a mechanism known as the hypothalamic-pituitary-ovarian axis. This mechanism controls and regulates female reproduction, including the timing of your menstrual cycle and is influenced by environmental factors like stress. 

They believe the dosing schedule for the two-dose mRNA vaccines—no matter if you chose the Moderna or Pfizer-BioNTech vaccine—supports this theory. “Given the dosing schedule of the mRNA COVID-19 vaccines in the United States [21 days for Pfizer and 28 days for Moderna], an individual receiving two doses in a single cycle would have received the first dose in the early follicular phase. Cycle length variability results from events leading to the recruitment and maturation of the dominant follicle during the follicular phase, processes known to be affected by stress,” write the researchers.

While the researchers made no suggestion in regards to which menstrual phase was best to get your vaccine, they did find that if you're scheduled to get your first dose during your follicular phase (a phase that begins on the first day of your period and lasts for 10 to 17 days), you may see temporary delays to your menstrual cycle. This timing might also mean that you’d receive two doses within the same menstrual cycle, further contributing toward potential delays.

Study limitations to consider

One limitation to the study is that the results may not be generalizable to the diverse population in the U.S. For example, people who took part in the study were more likely to be white, college-educated and with lower BMIs than the national average.

The researchers also biased their pool of participants by including only people with regular menstrual cycles. However, some women frequently experience irregular menstrual cycles. It remains unknown how people with generally irregular menstrual cycles are affected by Covid vaccinations. 

Sources

Edelman A, et al. Association Between Menstrual Cycle Length and Coronavirus Disease 2019 (COVID-19) Vaccination. Obstetrics & Gynecology. 10.1097/AOG.0000000000004695 (2022): doi:10.1097/AOG.0000000000004695